1. Field of the Invention
This invention relates to a method for producing an ebullient heat transfer pipe with an extremely high heat transfer rate in a facilitated and economical manner, the pipe being particularly suitable for use in boiling type heat exchangers.
2. Description of the Prior Art
In order to increase the heat transfer rate of a heat transfer pipe or a so-called boiling heat pipe which is resorted to for the purpose of effectively transferring heat to a liquid such as liquefied nitrogen, liquefied oxygen, alcohol, water or the like which is in contact with a pipe surface, it is necessary to improve the boiling heat transfer characteristics in a range of low temperature differences. Therefore, as shown in FIG. 1 (i.e. a sectional view in the axial direction of a heat transfer pipe), it has been the general practice in the manufacture of boiling heat pipes to work the pipes such that in the final stage each pipe is provided with a multitude of openings (or gaps) a on the pipe surface and cavities (or tunnels) b which communicate the respective openings in the circumferential direction. In FIG. 1, reference character c denotes a released bubble, and reference character d a bubble which has been left in a cavity b. The pipe is desired to be able to hold part of the generated bubbles as much as possible in the cavities b from the standpoint of accelerating ebullition of a liquid and enhancing the heat transfer rate.
Following are three typical methods which have thus far been used in the art for the production of pipes of this sort.
(1) A method of forming a porous layer on the surface of a pipe by sintering fine metal particles thereon.
(2) A method of ploughing out a helical fin of saw-toothed shape on the surface of a pipe by the use of a cutting tool and then bending the wavy fin in an axial direction of the pipe by a wire brush to form a helical tunnel-like cavity which is intermittently covered with the bent fin portions.
(3) A method of breaking upper side portions of a fin by plastic deformation in a conventional fin-forming process and laterally bulging out shoulder portions of the respective upper fin portions by compressive deformation to thereby form a tunnel-like cavity with openings at certain intervals, followed by partial compressive deformation of the fins for deforming the openings of the cavity into wide and narrow openings.
A pipe which is obtained by method (1) contains three-dimensionally distributed fine pores forming a large number of bubble generating nuclei and communicating with each other in various directions, such that it has an advantage in that a liquid can be continuously supplied to pores of active ebullition from pores of inactive ebullition through intercommunicating passages to accelerate the boiling heat transfer with a suitable bubble holding and developing effect. However, the method (1) which involves a sintering process inevitably suffers from low productivity and high production cost. In addition, it is difficult to form pores of uniform diameters owing to a difficulty of securing metal particles of uniform sizes and to ensure the above-mentioned bubble holding and developing effect occurring over the entire length of a pipe.
The pipes which are obtained by methods (2) and (3) have the openings communicated one-dimensionally by a helical cavity which extends in the circumferential direction of a pipe, such that they have a common problem in that a shortage of liquid supply occurs upon activation of ebullition. Accordingly, such are limited in the effect of accelerating the boiling heat transfer, and inferior in the above-mentioned bubble holding effect, coupled with a low productivity in the case of method (2) in which a fin is ploughed out by means of a cutting tool (e.g., a cutter) and which involves the possibility of fracturing pipes due to enlargement of fine cracks occurring in the fin-ploughing stage. Further, it is necessary in the case of method (3) to effect the compressive deformation within a range free of buckling of fins, but it is extremely difficult to carry out the compressive deformation in good condition and the manufacturing process requires complicated and costly equipment.